My 1st Edition AD&D DMG was sitting on my shelf, calling my name. Courtesy of the random encounter tables that fill the back of this excellent tome, I give you three random dungeon encounters. I'll roll a d12 to determine the level of the dungeon, then follow the Dungeon Random Monster Level Determination Matrix to determine the content of the encounter.
Encounter One (dungeon level: 12)
The die roll points me to the level IX table, where I roll 43, Yellow Mold! Adjusting for level difference of the table vs. dungeon level, I'd say this is a huge colony, large enough to potentially have formed a collective intelligence. A die roll of 1 on a d6 indicates that yes, this colony is sentient, meaning it can sense approaching entities and release a spore cloud offensively, and use psionic attacks if it detects psionic use nearby.
I would set up this colony in a large open area in a cavern or passage, where creatures passing through can serve as a food source. The floor of this passage would be scattered with the remains of past victims, along with any surviving equipment they might have been carrying. Yellow Mold doesn't affect metal so coins, armor and weapons would remain, long after flesh and other organics have been consumed.
Encounter Two (dungeon level: 3)
The d20 roll comes up 12, indicating monster level III, and the subsequent 51 indicates 2-5 Wererats! A d4 roll comes up 1, indicating 2 Wererats total.
I'd play this as two clever Wererats intent on luring the party into a trap or splitting up the members. Wererats can take human form so infiltration is one possibility. Using rat form to spy on the group and gather information about their capabilities is another. I'd also establish a small lair with rat- and human-sized exits for the Wererats. The creatures might have a small cache of supplies and treasure hidden away in their lair, but protected by one or more traps.
Encounter Three (dungeon level: 9)
The die roll directs me to monster table VII, and the percentile roll there comes up 67 a Mummy (only one based on the subsequent roll).
Clearly the party has stumbled into a long abandoned crypt where a defiled grave has released this horrible foe. It roams the corridors of its tomb, unleashing its undying hatred of the living on anything it encounters. Unfortunately for the party, the grave-robbers that released the creature successfully made off with any loot buried in the tomb.
Who said random encounters weren't fun?